Hi, I'm getting very decent results with chrony and a usb gps.
According to the statistics the std dev is around 1ms, which I might
hope is slightly too high and that as low as 0.5ms is feasible? I think
that this might indicate that we can do better measuring the arrival
time of the $ in the NMEA sentence, but now I need to collect some
evidence for that

So my theory is that gpsd is measuring the timestamp of the end of the
ZDA sentence (and working that back to get the initial timestamp) and
this gives an additional 0.5-1ms jitter that might not be there if we
measured the timestamp of the initial $. If so then I might expect to
see a jitter distribution which is positively skewed (every time the
last character drops into a later 1ms read)

Can someone give some guidance on the correct statistics to plot to
investigate this further? I'm a little unsure on how to interpret the
stats in /var/log/statistics.log also /var/log/tracking.log seems
interesting?

On a related note, did anyone ever try and improve the time accuracy of
USB serial reading by noting that serial jitter is lower than USB jitter
and trying to interpolate character arrival time vs USB timestamps to
try and figure out a kind of subsample accuracy? ie every time you read
from the USB, note the timestamp and how many characters read - given
that the usb freq isn't the same as the serial freq we might be able to
improve our estimate of the exact time the intial $ character was emitted?

Thanks
Ed W
--

To unsubscribe email chrony-users-request@xxxxxxxxxxxxxxxxxxxx
with "unsubscribe" in the subject.
For help email chrony-users-request@xxxxxxxxxxxxxxxxxxxx
with "help" in the subject.